As a device for controlling rotation of a motor, there is known an absolute encoder device. The absolute encoder device can be used for, for example, detecting a rotation direction, a rotational frequency, or a rotational position of a motor. As this absolute encoder device, there are a magnetic type and an optical type. The magnetic type is less expensive than the optical type and is superior in environmental resistance.
In the magnetic type absolute encoder device, for example, a magnetic sensor is arranged to be opposed to a bipolar permanent magnet fixed to a rotation shaft. As the magnetic sensor, there is known a sensor using a spin-valve giant magnetoresistive element (SV-GMR) as a magnetoresistive element (for example, see Patent Literature 1). The SV-GMR type magnetic sensor includes a pinned layer and a free layer. A pinned layer magnetization direction is fixed, and a free layer magnetization direction changes in accordance with an external magnetic field direction. A resistance change in the free layer and the pinned layer produces outputs of sine wave signals having different phases when the rotation shaft rotates one turn. However, when the SV-GMR type magnetic sensor is used for detecting rotation, the free layer magnetization direction rotates in accordance with the external magnetic field. Therefore, it is not possible to completely saturate the magnetic field of the free layer without affecting a magnetization direction of the pinned layer that is used as a reference direction or coupling of layers (magnetostatic coupling or the like). Therefore, the operation is performed in the external magnetic field under a saturated magnetic field, and hence a resistance change ratio is limited.
In addition, as another magnetic sensor, there is known a sensor including a bias magnet for an anisotropic magnetoresistive element (AMR) as the magnetoresistive element (for example, see Patent Literature 2). This AMR type magnetic sensor uses a magnetic field of a stable permanent magnet as a magnetization direction in the reference direction and has a single layer, and therefore does not affect coupling of layers. However, there are such problems that the resistance change ratio is lower than that of the SV-GMR type magnetic sensor. Therefore, it is difficult to obtain high resolution and high accuracy regardless of the type of the magnetic sensor in the absolute encoder device using only the bipolar permanent magnet.
In view of this technical background, there is also known a device that aims to achieve higher resolution and higher accuracy of the encoder device not by the type of the sensor but by a configuration of the permanent magnet or the like (for example, see Patent Literatures 3 and 4). In the encoder device of Patent Literature 3, a rotational position detecting magnet (PG magnet) is arranged on an outer peripheral surface of a rotating substrate formed into a disc shape, and a magnetic pole position detecting magnet (pole magnet) is arranged on a top surface of the rotating substrate. In addition, in the encoder device of Patent Literature 4, a first track magnetized with a plurality of poles is formed on an outer peripheral surface of the disc-shaped rotator, and a second track magnetized with a single pole is formed on a lower surface of the disc-shaped rotator. In this way, in the devices of Patent Literatures 3 and 4, two types of magnetic patterns are combined so that higher resolution and higher accuracy can be obtained than in the absolute encoder device using only the bipolar permanent magnet.
However, in the device of Patent Literature 3, a substrate or a support member for supporting a position detecting element (MR element) for detecting a magnetic field from the rotational position magnet is necessary in addition to a substrate for supporting a magnetic pole position detecting element (Hall element) for detecting a magnetic field from the magnetic pole position detecting magnet. In addition, because the magnetic pole position detecting magnet that generates a strong magnetic field is arranged on a top surface of the rotating substrate, the magnetic field from the magnetic pole position detecting magnet affects the magnetic field of the rotational position magnet and the position detecting element for detecting a magnetic field from this magnet. As a solution to this problem, in Patent Literature 3, a magnetic shield plate as a separate member is arranged between the magnetic pole position detecting magnet and the rotational position magnet.
Also in the encoder device of Patent Literature 4, because the second track that generates a strong magnetic field is arranged on the lower surface of the disc-shaped rotator, a magnetic field from the second track affects a magnetic field of the first track and a magnetic detecting element for detecting the magnetic field from the first track. In addition, a support member for supporting the magnetic detecting element for detecting the magnetic field from the first track is necessary in addition to a substrate for supporting the magnetic detecting element for detecting the magnetic field from the second track.
In this way, in each of the devices of Patent Literatures 3 and 4, separate members are necessary to support the two magnetic sensors, and these separate members need to be assembled in consideration of a distance to the magnet and a distance between the sensors. Therefore, adjustment in assembling is necessary, and cost is increased. Further, because one of the magnetic patterns is formed on the outer peripheral surface of the disc-shaped rotator while the other magnetic pattern is formed on the top surface or the lower surface of the disc-shaped rotator, there is a problem of interference due to a leakage magnetic field between the magnetic patterns. In order to solve this problem, if the magnetic shield plate is arranged, the number of components is further increased, and hence difficulty in assembling and cost are increased.
In addition, in the device of Patent Literature 3, the magnetic pole position detecting element detects a power supply position to a driving coil of the motor by approximately a few pulses per rotation based on a magnetic field from the magnetic pole position detecting magnet. In the device of Patent Literature 4, the magnetic detecting element for detecting the magnetic field from the second track outputs one index signal, namely Z-phase signal, per rotation. In order for the devices of Patent Literatures 3 and 4 to function as absolute encoder devices using this output signal, it is necessary to perform a complicated calculation process of an incremental output signal obtained from the position detecting element (Patent Literature 3) for detecting the magnetic field from the rotational position magnet or from the magnetic detecting element (Patent Literature 4) for detecting the magnetic field from the first track. Therefore, it is necessary to arrange a separate component for this calculation process. For instance, in the device of Patent Literature 4, in order to keep necessary multiple rotation information in an absolute specification, a counter and a battery are arranged. In addition, the counter and the battery are necessary also for the device of Patent Literature 4 to function as the absolute encoder device for detecting an absolute position within one turn.